To reproduce:
1. Open the "demo_scene" in the user-provided project
2. In the Lighting window, select "Clear Baked data"
3. Press "Generate Lighting"

Desired outcome: After the baking process is done, there are no artifacts behind the Game Object with open geometry ("demo_gate")
Actual result: During the baking process and after it is done, there are artifacts behind the Game Object with open geometry ("demo_gate")

Reproduced with 5.6.1f1, 5.6.3p1, 2017.1.0a1, 2017.1.0p4, 2017.2.0a1, 2017.3.0a5
Not reproduced with 2017.2.0b8